In order to reduce atmospheric pollution caused by the emission of potentially harmful substances from engines, legislation has been introduced in the United States of America (USA) and the European Union (EU) to progressively lower legally binding limits for certain emissions, including oxides of nitrogen (NOx).
Under US legislation, known as the “Tier 4 Final” standard, and the EU Stage IV legislation, emissions of NOx are limited to 0.40 g/kWh for engines with power outputs in the range of 56 to 560 kW.
Various emission abatements technologies are known for reducing the output of NOx from diesel engines, including exhaust gas recirculation (EGR) systems, which inhibit NOx production by lowering the combustion temperature, lean NOx traps (LNT) or NOx adsorber catalysts (NAC) which act to ‘hold’ NOx, and selective catalytic reduction (SCR) which converts NO and NO2 to nitrogen and water.
SCR combines the use of a catalyst such as vanadium, tungsten, copper zeolite (Cu-Zeolite) or iron zeolite (Fe-Zeolite) with a reductant such as anhydrous ammonia, aqueous ammonia or, more typically, urea, to convert NO and NO2 into nitrogen and water. Urea is typically used as the reductant, but has to be injected into the exhaust upstream of the SCR catalyst in order to thermally decompose into ammonia by the point at which it enters the SCR catalyst. Urea is preferred over ammonia, as it is substantially safer to store and transport. In the USA, commercially available urea for use with SCRs is referred to as Diesel Exhaust Fluid (DEF), whereas in Europe it is referred to as AdBlue®. For SCRs to function effectively at the lower end of the temperature spectrum it has hitherto been desirable for there to be a 50:50 split of NO and NO2, although Cu-Zeolite catalysts have been found to improve performance at temperatures of less than 300° C. when there is little NO2 available. An advantage of SCR is that is has minimal impact on specific fuel consumption. Disadvantages include the need to additionally replenish the reductant on a periodic basis and to provide space on a vehicle to package a reservoir of reductant. Typically, reductant usage is 1-7% that of diesel consumption.
The urea solution is injected into the exhaust passage using a dosing module, also known as a DEF or AdBlue® injector or a dosing valve. The formation of crystals of urea within a dosing module adversely affects the supply of urea to the exhaust passage, for example the formation of urea crystals at the dosing module tip (orifice crystallization) may prevent the injection of urea out of the dosing module and the formation of urea crystals within the body of the dosing module (in-valve crystallization) may prevent movement of the valve and thus prevent the supply of urea.
A known method for unblocking dosing modules is to increase the temperature of the dosing module and melt the urea crystals using exhaust gases. This requires the temperature of the dosing module to be heated to over 100° C. This is difficult to achieve since only the tip of the dosing module is located close to the exhaust gases and the body of the dosing module is surrounded by a coolant jacket. Furthermore, on a low level duty cycle, the exhaust gas temperature is not high enough raise the temperature of the dosing module to a level sufficient to melt urea crystals.